1. Field of the Invention
The present invention relates to a tracking error detection method allowing a stable tracking servo to be achieved in an optical disc reproduction apparatus even during reproduction of data on a high density disc or during high-speed reproduction, and relates to an optical disc reproduction apparatus using the method.
2. Description of the Prior Art
An optical disc known as a CD (Compact Disc) and a DVD (Digital Versatile Disc) is an information recording medium which is used for recording and reproducing information by means of an optical spot formed by concentrating laser light on the information recording surface of the disc. A BD (Blu-ray Disc) or an HD DVD is one of discs that have recently put into practical use as a high density optical disc. The Blu-ray Disc is described in details in “White Paper: Blu-ray Disc Format-General,” and the HD DVD is described in details in “DVD Forum Gijyutsu Hakusho—HD DVD Format no Gaiyou—(DVD Forum Technology White Paper—Outline of HD DVD Format—).”
In a case of a reproduction-only optical disc (ROM: Read Only Memory), information is recorded in the form of fine convexoconcaves (pits) formed on the information recording surface, and the information is reproduced by detecting the change in the reflectance of light caused by the presence and absence of pits. The pits are aligned spirally or concentrically, and thus form a recording track.
When information is reproduced, it is necessary to cause the optical spot to follow a recording track precisely (perform tracking). The tracking is performed by causing optical means to detect the amount of deviation (a tracking error) of the optical sport from the center of a recording track, and by driving an objective lens in a radial direction of the disc so that the tracking error would be zero.
There are a 3-spot method, a DPD (Differential Phase Detection) method and the like as a tracking error detection method for reproduction-only optical discs. The DPD method is often used in recent driver apparatuses in order to handle a plurality of types of disc standards, since the DPD method can eliminate the influence of the difference between track pitches.
A fundamental principle of the tracking error detection using the conventional DPD method will be described by using FIG. 1. FIG. 1 shows states of pits, optical spots, and light intensity distribution on a quadrant photo detector when information in a reproduction-only disc is reproduced. When an optical spot is on a pit, the 0th-order diffracted light and the ±1st-order diffracted light among all the orders of diffracted lights generated by the pit interfere with each other, and this interference generates areas (interference areas) with the small light intensity on the quadrant photo detector.
In this respect, assuming that segment elements of the quadrant photo detector are A, B, C and D, an examination is given on the sums (IA+IC) and (IB+ID) each of which is the sum of the output currents from two segment elements located at diagonal corners, respectively. In a case where the optical spot passes exactly through the center of the pit column, no phase difference occurs between (IA+IC) and (IB+ID). On the other hand, when the optical spot is deviated from the centerline of the pit column (off-track), a phase difference occurs between (IA+IC) and (IB+ID). This is because the above-described interference areas are generated as a pair at the respective diagonal corners of the quadrant photo detector, when the optical spot passes on an edge of the pit (a front or rear end of the pit in the direction of optical spot's movement). As shown in FIG. 1, the sign of phase difference varies according to off-track directions, and the size of phase difference is approximately proportional to the off-track amount.
Hereinafter, an example of the tracking error signal generating method using the conventional DPD method will be described by using a block diagram of FIG. 2 and a diagram of signal waves of FIG. 3. The diagonal sum signals (IA+IC) and (IB+ID) are amplified by the respective amplifiers, and thus become A1 and A2, respectively. The amplitudes of short mark signals of the A1 and A2 are relatively amplified by the respective equalizers. Subsequently, the signals are binarized by the respective level comparators, and thus become signals B1 and B2, respectively. The signals B1 and B2 are inputted to a phase comparator, and the phase comparator detects the phase difference between edges of the two signals (an edge is a point where each of the signals crosses over the zero level). The phase comparator outputs a phase difference pulse to C1 when the phase of the signal B1 precedes the phase of the signal B2. On the other hand, the phase comparator outputs a phase difference pulse to C2 when the phase of the signal B2 precedes the phase of the signal B1. The heights of the phase difference pulses are constant, and the widths thereof are equal to the absolute value of the phase difference between the signals B1 and B2. The pulse signals of each of C1 and C2 are integrated with a predetermined time constant by the respective low-pass filters (LPF), and the difference between the signals thus obtained becomes a tracking error signal.